1. The trait is sex-linked and recessive.
2. In generation I, individuals 1 and 4 are carriers (they are unaffected females who have affected sons, indicating they carry the recessive allele on the X chromosome). In generation II, individuals 2 and 4 are carriers (they are unaffected females with affected sons or daughters, indicating they carry the recessive allele).
3. We can't determine who are carriers in generation III because all individuals in generation III are either unaffected or affected, and the trait is recessive and sex-linked. Since males only have one X chromosome, if they are affected, they must have inherited the recessive allele from their mother. Females in generation III who are unaffected could be either homozygous dominant or heterozygous carriers, but without further information (such as their offspring), we cannot determine their carrier status.
4. The second-generation children inherit sickle cell anemia from their mother. In both families, the affected males in generation II (II-2 and II-5) have unaffected mothers (I-1 and I-4), indicating that the mothers are carriers of the recessive allele on the X chromosome. The affected males received the X chromosome with the recessive allele from their mothers.
5. Carriers are only depicted in pedigrees that include recessive traits because carriers are individuals who have one copy of the recessive allele but do not show the trait. In pedigrees with dominant traits, individuals with the trait are either homozygous dominant or heterozygous, and heterozygous individuals show the trait, so there is no need to distinguish carriers.
Parent Tip: Review the logic above to help your child master the concept of pedigree practice worksheet.
